High pressure polymorphism of LiBH₄ and of NaBH₄

The pressure-induced structural changes in LiBH₄ and in NaBH₄ have been investigated experimentally up to 290 GPa by coupling Raman spectroscopy, infrared absorption spectroscopy and synchrotron X-ray diffraction. This data set is also analysed in the light of Density Functional Theory calculations performed up to 600 GPa. The [BH₄]^- unit appears to be remarkably resistant under pressure. NaBH₄ remains stable in the known Pnma γ-phase up to 200 GPa and calculations predict a transition to a metallic polymeric C2/c phase at about 480 GPa. LiBH₄ is confirmed to exhibit a richer polymorphism. A new Pnma orthorhombic phase VI is found to be stable above 60 GPa and there are hints of a possible phase VII above 160 GPa. DFT calculations predict that two other high pressure LiBH₄ phases should appear at about 290 and 428 GPa. A very slight solubility of H₂ inside phases II, III and V of LiBH₄ is observed. A NaBH₄(H₂)_0.5 complex is predicted to be stable above 150 GPa.